As the use and complexity of digital circuits has increased in approximately the last decade, so also has the need to test such circuits increased in order to ensure proper operation. Basically, two types of digital circuit test techniques have been developed, namely functional test techniques and so-called in-circuit test techniques.
In functional test techniques a known digital pattern is applied to the circuit input and a comparison is made of the circuit output with an expected output. The differences between the actual and expected outputs provides an indication of circuit operation. Unfortunately, this technique is only useful when it is desirable to know the overall operation of a circuit, for example a circuit which has been assembled onto a printed circuit board. Very often it is desirable to test individual circuit elements or groups of elements which have been assembled onto a printed circuit board apart from the overall circuit operation.
In in-circuit testing techniques, testing is performed on a circuit element or elements isolated from the remainder of the circuit. In-circuit testing techniques generally involve the application of a preselected digital pattern to the input of an individual circuit element, a so-called device under test (DUT), and the comparison of the DUT response to an expected response. Since the circuit element or elements under test typically are connected to other circuit elements, it is required to overdrive any digital pattern or signal which is being applied by an "upstream" circuit element or logic device. Upstream logic devices are those devices whose outputs normally drive the inputs of the DUT. An overdrive signal is a signal which is superimposed at a selected location in a circuit.
In relation to such in-circuit testing several devices have been described. U.S. Pat. No. 3,781,689 discusses a pulse generator which provides three output states for use in an in-circuit probe, namely logic high, logic low and an "off" state, the so-called three-state or TRI-STATE.
In order to perform multiple simultaneous in-circuit tests on several individual circuit elements mounted on a single printed circuit board, test devices such as that disclosed in U.S. Pat. No. 4,588,945 were developed. In such devices a printed circuit board having circuit elements mounted thereon is in turn mounted or affixed to a so-called bed of nails. Each nail acts as an individual probe either providing a preselected signal to or receiving an output signal from a DUT. As described in that patent, a controller module applies multiple pregenerated signal patterns to multiple DUTs through a driver module. The DUT responses are received through a sensor module and compared to expected responses. As discussed in that patent, the driver module is made up of a multiplicity of identical driver circuits which generate the actual voltage signals provided to selected probes or nails in the test bed or bed of nails. These circuits each provide logic high, logic low and an "off" state, i.e. the so-called tri-state.
Two major problems, however, existed with such prior art driver circuit technology: problems involving (1) too high an output impedance during logic high and logic low states, resulting in poor voltage accuracy at high currents, and (2) limited speed. Regarding the first problem, as a load on the driver circuit becomes greater (higher output current) a high output impedance will force the error component present in the output voltage to be greater than it would be with a lower output impedance. Thus, prior art drivers, for both in-circuit and functional testing, exhibited both high output impedance and low voltage accuracy at the DUT. In addition, prior art drivers for in-circuit testing are relatively slow, lacking the speed required for functional testing.